Underground coal gasification (UCG) is a process by which product gas is produced from a coal seam by combusting and gasifying the coal in situ in the presence of an oxidant. The product gas is typically referred to as synthesis gas or syngas and can be used as a feedstock for electricity or chemical production, for example.
Conversion of coal into product gas takes place in a well which typically comprises a channel of sorts extending through the coal seam. Such a channel can be formed by one or more bore holes drilled into the coal seam that are in fluid communication with one another or by non-drilling methods as described below. For UCG, the channel is also in fluid communication with an injection well and a production well.
A coal seam panel is typically referred to as a coal gasifier. Gasification occurs adjacent a combustion zone of the well/gasifier and the coal is partially oxidized to produce product gas of low or medium heating value. Hot product gas flows from the gasification zone and exits the ground from a well head of the production well. As coal is consumed or gasified, a gasifier cavity within the coal seam develops and grows in size.
Non-drilling methods for permeabilizing coal and connecting (linking) wells together by way of a linking channel such that they are in fluid communication with one another are known. Such methods utilize chemical, electrical, thermal or mechanical forces or combinations of these. Typically, coal of the coal seam is permeabilized outwardly from a base of a vertical or inclined well and a horizontal linking channel of sorts is formed to another well or other type of open zone in the coal seam (eg. a gasifier cavity).
One known method of creating a linking channel involves burning through the coal seam using only the natural permeability of the coal. Burning through of a channel can also be performed after artificially increasing the coal seam's permeability by heat treatment, hydraulic or pneumatic rupture.
A known method of increasing the permeability of a massif of mineral deposits is by hydraulic rupturing whereby liquid is injected under high pressure into the massif from the base of a well. However, a disadvantage of the method is that water injected into a UCG gasifier cavity may have a quenching effect.
Another known method of creating a horizontal linking channel between a well and an operational gasifier cavity involves operating a high-pressure water jet at a base of a vertical well. Although the jet (hydro monitor) may target and penetrate the coal seam quite effectively, again, a disadvantage of the method is that water injected into the gasifier cavity may have a quenching effect.
Yet another linkage method concerns the injection of high pressure air (20-40 atm) so as to further open natural cracks/fractures in the coal seam. This method involves injecting pressurised air into the coal seam from a vertical well. A disadvantage of this method, however, is that pressurized air tends to preferentially act on existing cracks in the coal seam rather than propagate new cracks. Hence, typically cracks will occur in no more than about 15% of the total volume in the coal seam and linkage may thus not be successful. Another disadvantage is that, due to low permeability (filtration) of coal (shale), it is difficult to create a channel. This leads to low efficiency of the method. An associated disadvantage is that further vertical wells may need to be installed, which increases operating costs. Yet another disadvantage is the high energy expenditure required for the injection of large quantities of air to achieve linkage.